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We present a weak-lensing study of SPT-CLJ2040-4451 and IDCSJ1426+3508 atz=1.48 and 1.75, respectively. The two clusters were observed in our "SeeChange" program, a HST survey of 12 massive high-redshift clusters aimed athigh-z supernova measurements and weak-lensing estimation of accurate clustermasses. We detect weak but significant galaxy shape distortions using IR imagesfrom the WFC3, which has not yet been used for weak-lensing studies. Bothclusters appear to possess relaxed morphology in projected mass distribution,and their mass centroids agree nicely with those defined by both the galaxyluminosity and X-ray emission. Using an NFW profile, for which we assume thatthe mass is tightly correlated with the concentration parameter, we determinethe masses of SPT-CL J2040-4451 and IDCS J1426+3508 to beM_{200}=8.6_{-1.4}^{+1.7}x10^14 M_sun and 2.2_{-0.7}^{+1.1}x10^14 M_sun,respectively. The weak-lensing mass of SPT-CLJ2040-4451 shows that the clusteris clearly a rare object. Adopting the central value, the expected abundance ofsuch a massive cluster at z>1.48 is only ~0.07 in the parent 2500 sq. deg.survey. However, it is yet premature to claim that the presence of this clustercreates a serious tension with the current LCDM paradigm unless that tensionwill remain in future studies after marginalizing over many sources ofuncertainties such as the accuracy of the mass function and themass-concentration relation at the high mass end. The mass of IDCSJ1426+3508 isin excellent agreement with our previous ACS-based weak-lensing result whilethe much higher source density from our WFC3 imaging data makes the currentstatistical uncertainty ~40% smaller.

astrophysical journal/the astrophysical journal

First Weak-lensing Results from "See Change": Quantifying Dark Matter in  the Two Z>1.5 High-redshift Galaxy Clusters SPT-CL J2040-4451 and IDCS  J1426+3508